To isolate and purify specific hormone receptors in particular receptor tissues; to map function of the receptor against activation of adenylate cyclase; to determine the nature of the link between generation of cyclic 3', 5'-AMP and solute transport in particular cell systems. Beta-adrenergic receptors have been identified using a radioiodinated analog of a Beta-blocker designed for this purpose. Hydroxybenzylpindolol (1-(1-para-hydroxyphenyl-2-methyl-propylamino)-3(4-indolyloxy)-2-propanol) is a high-affinity Beta-blocker containing a phenolic group. The latter compound can be radioiodinated to high specific activity and binds specifically to Beta-receptor sites on cell membranes. Kinetic and equilibrium studies identified a single class of high affinity (2-4 x 10 to the 10th power M-1) receptor sites on turkey erythrocyte membranes, 200-400 sites per cell. A series of Beta-adrenergic agonists and inhibitors compete for this binding site with affinities paralleling biological effectiveness as activators or inhibitors of catecholamine-stimulated adenylate cyclase. Receptor occupancy for agonists correlated linearly with function (cyclic AMP production). The stereospecificity of interaction with the iodinated Beta-blocking agent and the correspondence between affinity for site and biological effectiveness indicate that binding of iodohydroxybenzylpindolol validly reflects interaction with the Beta-adrenergic receptor.